There has been a significant movement toward developing and performing cardiovascular surgeries using a percutaneous approach. As used herein, the term “percutaneous” is defined as an alternative to a surgical approach whereby one or more catheters are introduced into the body via a small puncture, and typically into a body lumen, for example, the femoral artery. Through the one or more catheters, tools and devices can be delivered to a desired area, such as in the cardiovascular system, to perform any number of complicated procedures that normally otherwise require an invasive surgical procedure. Such approaches greatly reduce the trauma endured by the patient and can significantly reduce recovery periods. The percutaneous approach is particularly attractive as an alternative to performing open-heart surgery.
Valve replacement surgery provides one example of an area where percutaneous solutions are being developed. A number of diseases result in a thickening, and subsequent immobility or reduced mobility, of heart valve leaflets. Such immobility also may lead to a narrowing, or stenosis, of the passageway through the valve. The increased resistance to blood flow that a stenosed valve presents can eventually lead to heart failure and ultimately death.
Treating valve stenosis or regurgitation has historically involved complete removal of the existing native valve through an open-heart procedure followed by the implantation of a prosthetic valve. Naturally, this is a heavily invasive procedure and inflicts great trauma on the body leading usually to great discomfort and considerable recovery time. It is also a sophisticated procedure that requires great expertise and talent to perform.
Recently, however, much attention has been given to the development of replacement heart valves that can be delivered percutaneously through a catheter. Understandably, size is a key design consideration when developing a device that is catheter-deliverable. The device must either be smaller than the lumen of a catheter, or able to be compressed until it is. In the case of a prosthetic valve, the size of the valve is determined by the native valve being replaced, or at least by the vasculature into which the percutaneous valve is being placed. A valve that is too small will likely fail to meet the demand for blood through the vessel and act as a source for future stenosis. The catheter delivering the valve will necessarily have to be smaller than the targeted blood vessel. As such, percutaneously-delivered heart valves must be capable of being compressed and loaded into a catheter, and subsequently expanded upon delivery.
Percutaneously-delivered heart valves must also, and arguably most importantly, be able to withstand the rigors of being repetitively opened and closed during use. Considering a heart, beating at an average of 70 beats per minute, beats over 100,000 times a day, any flaw in a prosthetic valve, whether it be a design flaw or a mechanical flaw, will greatly reduce the lifespan of the valve and potentially its user.
Hence, designing a valve that is both rugged enough for long-term use, and compressible enough to be placed into a catheter, is a daunting task. One promising design is shown and described in U.S. patent application Ser. No. 11/443,814, entitled Stentless Support Structure by Thill et al. and is incorporated by reference herein in its entirety. In short, the embodiments shown in this application include a support structure 5 (see, e.g. FIG. 1) and a valve 10. The support structure 5 is preferably a braided tube made out of Nitinol or a similar material. The braided tube folds in on itself one or more times upon deployment to multiply its radial strength.
The valve 10 is constructed to mimic a native valve and generally comprises one or more sheets of porcine tissue 12 attached to a commissural wireform 14. The wireform 14 gives the tissue 12 the correct shape in order to form leaflets that coapt during diastole and separate during systole. The tissue 12 is carefully sewn or otherwise attached to the support wire 14 such that, over time, the tissue does not tear or separate from the wireform 14. As such the wireform 14 flexes back and forth as the valve tissue 12 opens and closes.
Movement of the wireform 14 during cardiac function will cause the materials of a prosthetic valve to fatigue. The components of the prosthetic device should be able to withstand the expected loads and material cycles to which it will be subjected in the human cardiovascular system. Because the integrity of the wireform 14 affects the appropriate function of the valve 10, it would thus be desirable to modify the wireform 14 in order to improve the performance of the valve 10.